/*
 * @(#)ServiceLoader.java   1.10 06/04/10
 *
 * Copyright 2006 Sun Microsystems, Inc. All rights reserved.
 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

package com.mscg.jmp3.util.service;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.URL;
import java.util.ArrayList;
import java.util.Enumeration;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.NoSuchElementException;

/**
 * A simple service-provider loading facility.
 *
 * <p>
 * A <i>service</i> is a well-known set of interfaces and (usually abstract)
 * classes. A <i>service provider</i> is a specific implementation of a service.
 * The classes in a provider typically implement the interfaces and subclass the
 * classes defined in the service itself. Service providers can be installed in
 * an implementation of the Java platform in the form of extensions, that is,
 * jar files placed into any of the usual extension directories. Providers can
 * also be made available by adding them to the application's class path or by
 * some other platform-specific means.
 *
 * <p>
 * For the purpose of loading, a service is represented by a single type, that
 * is, a single interface or abstract class. (A concrete class can be used, but
 * this is not recommended.) A provider of a given service contains one or more
 * concrete classes that extend this <i>service type</i> with data and code
 * specific to the provider. The <i>provider class</i> is typically not the
 * entire provider itself but rather a proxy which contains enough information
 * to decide whether the provider is able to satisfy a particular request
 * together with code that can create the actual provider on demand. The details
 * of provider classes tend to be highly service-specific; no single class or
 * interface could possibly unify them, so no such type is defined here. The
 * only requirement enforced by this facility is that provider classes must have
 * a zero-argument constructor so that they can be instantiated during loading.
 *
 * <p>
 * <a name="format"> A service provider is identified by placing a
 * <i>provider-configuration file</i> in the resource directory
 * <tt>META-INF/services</tt>. The file's name is the fully-qualified <a
 * href="../lang/ClassLoader.html#name">binary name</a> of the service's type.
 * The file contains a list of fully-qualified binary names of concrete provider
 * classes, one per line. Space and tab characters surrounding each name, as
 * well as blank lines, are ignored. The comment character is <tt>'#'</tt> (
 * <tt>'&#92;u0023'</tt>, <font size="-1">NUMBER SIGN</font>); on each line all
 * characters following the first comment character are ignored. The file must
 * be encoded in UTF-8.
 *
 * <p>
 * If a particular concrete provider class is named in more than one
 * configuration file, or is named in the same configuration file more than
 * once, then the duplicates are ignored. The configuration file naming a
 * particular provider need not be in the same jar file or other distribution
 * unit as the provider itself. The provider must be accessible from the same
 * class loader that was initially queried to locate the configuration file;
 * note that this is not necessarily the class loader from which the file was
 * actually loaded.
 *
 * <p>
 * Providers are located and instantiated lazily, that is, on demand. A service
 * loader maintains a cache of the providers that have been loaded so far. Each
 * invocation of the {@link #iterator iterator} method returns an iterator that
 * first yields all of the elements of the cache, in instantiation order, and
 * then lazily locates and instantiates any remaining providers, adding each one
 * to the cache in turn. The cache can be cleared via the {@link #reload reload}
 * method.
 *
 * <p>
 * Service loaders always execute in the security context of the caller. Trusted
 * system code should typically invoke the methods in this class, and the
 * methods of the iterators which they return, from within a privileged security
 * context.
 *
 * <p>
 * Instances of this class are not safe for use by multiple concurrent threads.
 *
 * <p>
 * Unless otherwise specified, passing a <tt>null</tt> argument to any method in
 * this class will cause a {@link NullPointerException} to be thrown.
 *
 *
 * <p>
 * <span style="font-weight: bold; padding-right: 1em">Example</span> Suppose we
 * have a service type <tt>com.example.CodecSet</tt> which is intended to
 * represent sets of encoder/decoder pairs for some protocol. In this case it is
 * an abstract class with two abstract methods:
 *
 * <blockquote>
 *
 * <pre>
 * public abstract Encoder getEncoder(String encodingName);
 *
 * public abstract Decoder getDecoder(String encodingName);
 * </pre>
 *
 * </blockquote>
 *
 * Each method returns an appropriate object or <tt>null</tt> if the provider
 * does not support the given encoding. Typical providers support more than one
 * encoding.
 *
 * <p>
 * If <tt>com.example.impl.StandardCodecs</tt> is an implementation of the
 * <tt>CodecSet</tt> service then its jar file also contains a file named
 *
 * <blockquote>
 *
 * <pre>
 * META - INF / services / com.example.CodecSet
 * </pre>
 *
 * </blockquote>
 *
 * <p>
 * This file contains the single line:
 *
 * <blockquote>
 *
 * <pre>
 * com.example.impl.StandardCodecs    # Standard codecs
 * </pre>
 *
 * </blockquote>
 *
 * <p>
 * The <tt>CodecSet</tt> class creates and saves a single service instance at
 * initialization:
 *
 * <blockquote>
 *
 * <pre>
 * private static ServiceLoader&lt;CodecSet&gt; codecSetLoader = ServiceLoader.load(CodecSet.class);
 * </pre>
 *
 * </blockquote>
 *
 * <p>
 * To locate an encoder for a given encoding name it defines a static factory
 * method which iterates through the known and available providers, returning
 * only when it has located a suitable encoder or has run out of providers.
 *
 * <blockquote>
 *
 * <pre>
 * public static Encoder getEncoder(String encodingName) {
 *     for(CodecSet cp : codecSetLoader) {
 *         Encoder enc = cp.getEncoder(encodingName);
 *         if(enc != null)
 *             return enc;
 *     }
 *     return null;
 * }
 * </pre>
 *
 * </blockquote>
 *
 * <p>
 * A <tt>getDecoder</tt> method is defined similarly.
 *
 *
 * <p>
 * <span style="font-weight: bold; padding-right: 1em">Usage Note</span> If the
 * class path of a class loader that is used for provider loading includes
 * remote network URLs then those URLs will be dereferenced in the process of
 * searching for provider-configuration files.
 *
 * <p>
 * This activity is normal, although it may cause puzzling entries to be created
 * in web-server logs. If a web server is not configured correctly, however,
 * then this activity may cause the provider-loading algorithm to fail
 * spuriously.
 *
 * <p>
 * A web server should return an HTTP 404 (Not Found) response when a requested
 * resource does not exist. Sometimes, however, web servers are erroneously
 * configured to return an HTTP 200 (OK) response along with a helpful HTML
 * error page in such cases. This will cause a {@link ServiceConfigurationError}
 * to be thrown when this class attempts to parse the HTML page as a
 * provider-configuration file. The best solution to this problem is to fix the
 * misconfigured web server to return the correct response code (HTTP 404) along
 * with the HTML error page.
 *
 * @param <S>
 *            The type of the service to be loaded by this loader
 *
 * @author Mark Reinhold
 * @version 1.10, 06/04/10
 * @since 1.6
 */

public final class ServiceLoader<S> implements Iterable<S> {

    private static final String PREFIX = "META-INF/services/";

    // The class or interface representing the service being loaded
    private Class<S> service;

    // The class loader used to locate, load, and instantiate providers
    private ClassLoader loader;

    // Cached providers, in instantiation order
    private LinkedHashMap<String, S> providers = new LinkedHashMap<String, S>();

    // The current lazy-lookup iterator
    private LazyIterator lookupIterator;

    /**
     * Clear this loader's provider cache so that all providers will be
     * reloaded.
     *
     * <p>
     * After invoking this method, subsequent invocations of the
     * {@link #iterator() iterator} method will lazily look up and instantiate
     * providers from scratch, just as is done by a newly-created loader.
     *
     * <p>
     * This method is intended for use in situations in which new providers can
     * be installed into a running Java virtual machine.
     */
    public void reload() {
        providers.clear();
        lookupIterator = new LazyIterator(service, loader);
    }

    private ServiceLoader(Class<S> svc, ClassLoader cl) {
        service = svc;
        loader = cl;
        reload();
    }

    private static void fail(Class service, String msg, Throwable cause) throws ServiceConfigurationError {
        throw new ServiceConfigurationError(service.getName() + ": " + msg, cause);
    }

    private static void fail(Class service, String msg) throws ServiceConfigurationError {
        throw new ServiceConfigurationError(service.getName() + ": " + msg);
    }

    private static void fail(Class service, URL u, int line, String msg) throws ServiceConfigurationError {
        fail(service, u + ":" + line + ": " + msg);
    }

    // Parse a single line from the given configuration file, adding the name
    // on the line to the names list.
    //
    private int parseLine(Class service, URL u, BufferedReader r, int lc, List<String> names) throws IOException,
                                                                                             ServiceConfigurationError {
        String ln = r.readLine();
        if(ln == null) {
            return -1;
        }
        int ci = ln.indexOf('#');
        if(ci >= 0)
            ln = ln.substring(0, ci);
        ln = ln.trim();
        int n = ln.length();
        if(n != 0) {
            if((ln.indexOf(' ') >= 0) || (ln.indexOf('\t') >= 0))
                fail(service, u, lc, "Illegal configuration-file syntax");
            int cp = ln.codePointAt(0);
            if(!Character.isJavaIdentifierStart(cp))
                fail(service, u, lc, "Illegal provider-class name: " + ln);
            for(int i = Character.charCount(cp); i < n; i += Character.charCount(cp)) {
                cp = ln.codePointAt(i);
                if(!Character.isJavaIdentifierPart(cp) && (cp != '.'))
                    fail(service, u, lc, "Illegal provider-class name: " + ln);
            }
            if(!providers.containsKey(ln) && !names.contains(ln))
                names.add(ln);
        }
        return lc + 1;
    }

    // Parse the content of the given URL as a provider-configuration file.
    //
    // @param service
    // The service type for which providers are being sought;
    // used to construct error detail strings
    //
    // @param u
    // The URL naming the configuration file to be parsed
    //
    // @return A (possibly empty) iterator that will yield the provider-class
    // names in the given configuration file that are not yet members
    // of the returned set
    //
    // @throws ServiceConfigurationError
    // If an I/O error occurs while reading from the given URL, or
    // if a configuration-file format error is detected
    //
    private Iterator<String> parse(Class service, URL u) throws ServiceConfigurationError {
        InputStream in = null;
        BufferedReader r = null;
        ArrayList<String> names = new ArrayList<String>();
        try {
            in = u.openStream();
            r = new BufferedReader(new InputStreamReader(in, "utf-8"));
            int lc = 1;
            while((lc = parseLine(service, u, r, lc, names)) >= 0)
                ;
        } catch (IOException x) {
            fail(service, "Error reading configuration file", x);
        } finally {
            try {
                if(r != null)
                    r.close();
                if(in != null)
                    in.close();
            } catch (IOException y) {
                fail(service, "Error closing configuration file", y);
            }
        }
        return names.iterator();
    }

    // Private inner class implementing fully-lazy provider lookup
    //
    private class LazyIterator implements Iterator<S> {

        Class<S> service;
        ClassLoader loader;
        Enumeration<URL> configs = null;
        Iterator<String> pending = null;
        String nextName = null;

        private LazyIterator(Class<S> service, ClassLoader loader) {
            this.service = service;
            this.loader = loader;
        }

        public boolean hasNext() {
            if(nextName != null) {
                return true;
            }
            if(configs == null) {
                try {
                    String fullName = PREFIX + service.getName();
                    if(loader == null)
                        configs = ClassLoader.getSystemResources(fullName);
                    else
                        configs = loader.getResources(fullName);
                } catch (IOException x) {
                    fail(service, "Error locating configuration files", x);
                }
            }
            while((pending == null) || !pending.hasNext()) {
                if(!configs.hasMoreElements()) {
                    return false;
                }
                pending = parse(service, configs.nextElement());
            }
            nextName = pending.next();
            return true;
        }

        public S next() {
            if(!hasNext()) {
                throw new NoSuchElementException();
            }
            String cn = nextName;
            nextName = null;
            try {
                S p = service.cast(Class.forName(cn, true, loader).newInstance());
                providers.put(cn, p);
                return p;
            } catch (ClassNotFoundException x) {
                fail(service, "Provider " + cn + " not found");
            } catch (Throwable x) {
                fail(service, "Provider " + cn + " could not be instantiated: " + x, x);
            }
            throw new Error(); // This cannot happen
        }

        public void remove() {
            throw new UnsupportedOperationException();
        }

    }

    /**
     * Lazily loads the available providers of this loader's service.
     *
     * <p>
     * The iterator returned by this method first yields all of the elements of
     * the provider cache, in instantiation order. It then lazily loads and
     * instantiates any remaining providers, adding each one to the cache in
     * turn.
     *
     * <p>
     * To achieve laziness the actual work of parsing the available
     * provider-configuration files and instantiating providers must be done by
     * the iterator itself. Its {@link java.util.Iterator#hasNext hasNext} and
     * {@link java.util.Iterator#next next} methods can therefore throw a
     * {@link ServiceConfigurationError} if a provider-configuration file
     * violates the specified format, or if it names a provider class that
     * cannot be found and instantiated, or if the result of instantiating the
     * class is not assignable to the service type, or if any other kind of
     * exception or error is thrown as the next provider is located and
     * instantiated. To write robust code it is only necessary to catch
     * {@link ServiceConfigurationError} when using a service iterator.
     *
     * <p>
     * If such an error is thrown then subsequent invocations of the iterator
     * will make a best effort to locate and instantiate the next available
     * provider, but in general such recovery cannot be guaranteed.
     *
     * <blockquote style="font-size: smaller; line-height: 1.2"><span
     * style="padding-right: 1em; font-weight: bold">Design Note</span> Throwing
     * an error in these cases may seem extreme. The rationale for this behavior
     * is that a malformed provider-configuration file, like a malformed class
     * file, indicates a serious problem with the way the Java virtual machine
     * is configured or is being used. As such it is preferable to throw an
     * error rather than try to recover or, even worse, fail
     * silently.</blockquote>
     *
     * <p>
     * The iterator returned by this method does not support removal. Invoking
     * its {@link java.util.Iterator#remove() remove} method will cause an
     * {@link UnsupportedOperationException} to be thrown.
     *
     * @return An iterator that lazily loads providers for this loader's service
     */
    public Iterator<S> iterator() {
        return new Iterator<S>() {

            Iterator<Map.Entry<String, S>> knownProviders = providers.entrySet().iterator();

            public boolean hasNext() {
                if(knownProviders.hasNext())
                    return true;
                return lookupIterator.hasNext();
            }

            public S next() {
                if(knownProviders.hasNext())
                    return knownProviders.next().getValue();
                return lookupIterator.next();
            }

            public void remove() {
                throw new UnsupportedOperationException();
            }

        };
    }

    /**
     * Creates a new service loader for the given service type and class loader.
     *
     * @param service
     *            The interface or abstract class representing the service
     *
     * @param loader
     *            The class loader to be used to load provider-configuration
     *            files and provider classes, or <tt>null</tt> if the system
     *            class loader (or, failing that, the bootstrap class loader) is
     *            to be used
     *
     * @return A new service loader
     */
    public static <S> ServiceLoader<S> load(Class<S> service, ClassLoader loader) {
        return new ServiceLoader<S>(service, loader);
    }

    /**
     * Creates a new service loader for the given service type, using the
     * current thread's {@linkplain java.lang.Thread#getContextClassLoader
     * context class loader}.
     *
     * <p>
     * An invocation of this convenience method of the form
     *
     * <blockquote>
     *
     * <pre>
     * ServiceLoader.load(<i>service</i>)
     * </pre>
     *
     * </blockquote>
     *
     * is equivalent to
     *
     * <blockquote>
     *
     * <pre>
     * ServiceLoader.load(<i>service</i>,
     *                    Thread.currentThread().getContextClassLoader())
     * </pre>
     *
     * </blockquote>
     *
     * @param service
     *            The interface or abstract class representing the service
     *
     * @return A new service loader
     */
    public static <S> ServiceLoader<S> load(Class<S> service) {
        ClassLoader cl = Thread.currentThread().getContextClassLoader();
        return ServiceLoader.load(service, cl);
    }

    /**
     * Creates a new service loader for the given service type, using the
     * extension class loader.
     *
     * <p>
     * This convenience method simply locates the extension class loader, call
     * it <tt><i>extClassLoader</i></tt>, and then returns
     *
     * <blockquote>
     *
     * <pre>
     * ServiceLoader.load(<i>service</i>, <i>extClassLoader</i>)
     * </pre>
     *
     * </blockquote>
     *
     * <p>
     * If the extension class loader cannot be found then the system class
     * loader is used; if there is no system class loader then the bootstrap
     * class loader is used.
     *
     * <p>
     * This method is intended for use when only installed providers are
     * desired. The resulting service will only find and load providers that
     * have been installed into the current Java virtual machine; providers on
     * the application's class path will be ignored.
     *
     * @param service
     *            The interface or abstract class representing the service
     *
     * @return A new service loader
     */
    public static <S> ServiceLoader<S> loadInstalled(Class<S> service) {
        ClassLoader cl = ClassLoader.getSystemClassLoader();
        ClassLoader prev = null;
        while(cl != null) {
            prev = cl;
            cl = cl.getParent();
        }
        return ServiceLoader.load(service, prev);
    }

    /**
     * Returns a string describing this service.
     *
     * @return A descriptive string
     */
    public String toString() {
        return "java.util.ServiceLoader[" + service.getName() + "]";
    }

}
